Newbie question: accessing solar power when grid is down

Hi all, and thanks in advance for you patience with this question.

I have been doing quite a bit of research on solar power and do want a grid tie sytem. Makes the most sense for a number of reasons. I am not looking at 100% replacement of my current usage or anything that spectacular, just to augment with the option to grow later if it makes sense.

However, it has been a bit unclear what it would take to have access to that solar power in the event that the grid is down. Again, not 100% replacement but enough to:

1) access the power that is generated during the day
2) provide minimal battery backup overnight for a single medical device.

(For the record, loss of use of the medical device is not life threatening)

I guess my thought has been that if the grid is down and its sunny outside, why can't I access the power generated by the panels during the day?

So here are my questions:

1) I understand that it is not necessary to have a 1:1 match between the solar array output and the battery backup. Is this correct?

2) If I have a very small battery backup (lets say 2 midsized batteries...I only need a minimum of ~2 amps contnuous (200W device) overnight in an emergecy), and the solar array is much larger than required to charge these batteries, can I access that extra power during daylight hours? (This is the bit I am confused about)

Let me rephrase that question: If the grid is down and the sun is shining, am I drawing power from the batteries or the solar array? What limits the system during daylight, the battery capacity or the solar array output?

3) I have noted that the efficiency of a battery backup system is significanly less than a straight grid tie system. Is this still true if I am just keeping a very minimal battery backup? In other words, is the loss in efficiency due to the size of the battery charging or due to inherent losses in the MPPT charger system?

As noted above, I am not trying to have a uninterrupted power supply. The backup "circuits" would just be plugs I could access with extension cords. Frankly, its mostly a matter of having a bit of "emergency" power supply just in case.

I noted that the Outback FlexWare 500 has knockouts for a GFI outlet, which is what started my thinking on this. That outlet would meet my needs for "emergency" power and now its a question of what it would really take.

Now, if is going to cost thousands of dollars and a severe drop in efficiency, I guess I will drop it. It just seems odd that the current system designs don't seem to allow access to the power that is available. I assume this is due to regulations related to the intertie, but it would be cool to get access to the solar array in an emergency.

thanks in advance!

[BB.]

Well, your options are:

1. Grid Tie, no solar backup power
2. Off-Grid, no GT back to the grid
3. Hybrid, Grid Tie with Off-Grid backup
4. Home Brew:

• a. Configurable wiring of solar panels to switch to battery charging
• b. Connect "GT Inverter" to "Off-Grid Inverter"--GT will backfeed and charge batteries through Off-Grid Inverter.

Referring to above:

1. Cheapest and least maintenance costs. If you don't have many outages--do this, and get a generator (gas, natural gas, propane) for emergency backup
2. Certainly less efficient--makes most sense if no grid available
3. Hybrids are nice, should be more efficient than #2. Basically, a solar powered UPS with Grid Tie inverter.
4. Hombrew:

• a. can be done, switches and/or jack panel. Certainly easy to wire a part of your array (maybe 1/4 or so easily, more of the array requires more work and pre-planning). Issues with code and safety (depending on how you do it).
• b. Also can be done--but at this time, I don't see any good (safe and reliable) way of integrating with a battery charge controller (complex system design issue). Go with #3 if you need a "full" sized GT / Off-Grid system.

-Bill

Thanks for the reply....

I guess I am a little confused. If I just put many thousands of dollars of PV panels on my roof, I don't understand why I should go out and get a generator! Your answer is consistent with everything I have read, so I am not really questioning it. I just don't understand what it is about this setup that makes it so difficult/expensive to allow me access to the power the system is generating!

For instance, I have to assume that the inverter and associated control equipment is creating a nice, clean power output for the utility company when the sun is shining. Why can't I just have a "sunshine" plug that gives me access to this when the grid is down? I could plug a completely separate battery system into the array if the grid goes down....in my mind (small as it may be) this seems reasonable yet all the information says that this is either "impossible" or "expensive".

My question is in earnest. If there is a technical reason, I would really like to know it. It just makes little sense to me that I can't tap into that output when the grid is down without the expense and complexity of adding a battery backup in the middle.

[BB.]

Yes, there is a reason why the "average" GT Inverter turns off when the utility power is down...

Basically, a GT Inverter is the exact opposite vs an Off-Grid Inverter...

The OG Inverter, outputs exactly 120 VAC @ 60Hz (Regulated Output). The DC input to the OG Inverter is variable current (and voltage to a degree) that the OG Inverter just takes, what it needs to supply its AC output.

The GT Inverter, on the other hand, more or less regulates on the DC input. And the output voltage & frequency is set by the utility (think of the utility as being a giant "AC Battery" for all intents and purposes).

The GT Inverter, monitors the input voltage and available current from the solar panels using the equation P=I*V (power = current * voltage). What it does, is it looks for Pmax based on Imax and Vmax, many times a second--and transfers as much power as possible (and legal) to the AC Utility lines.

If you tried to run a GT Inverter without an AC voltage for it to follow--it will assume the AC Mains have a problem and will shut down for safety (prevent electricution of utility workers and such).

There are other issues too... One is that a GT Inverter, typically requires 200-600 VDC for operation. The Solar Charge Controllers, which power the battery bank and OG Inverters, typically use Vbatt+a few volts to ~140 VDC maximum... So you cannot just "swap" the panels from a GT Inverter to a Solar Charge Controller.

Another is that to be able to switch between AC Mains + GT Inverter to OG Inverter only--there needs to be an AC Transfer Switch (extra parts and money).

If you want a true hybrid system... The Xantrex XW series is a really nice system. But, it will cost a lot more to install vs the standard GT Inverter.

Got to go now--please feel free to ask more questions for when I return.

-Bill

[niel]

if the only thing you are worried about is a noncritical piece of medical equipment to be backed up, then go with a full gt system and place a battery and charger into it to run a inverter that will power that medical device for times when the power isn't available. all other methods and ways, as few as there are, would be overkill for that one thing to be powered. this sounds like it might be a cpap machine. we have had some conversation on that subject.

Originally Posted by BB.

Well, your options are:

1. Grid Tie, no solar backup power
2. Off-Grid, no GT back to the grid
3. Hybrid, Grid Tie with Off-Grid backup
4. Home Brew:

• a. Configurable wiring of solar panels to switch to battery charging
• b. Connect "GT Inverter" to "Off-Grid Inverter"--GT will backfeed and charge batteries through Off-Grid Inverter.

Referring to above:

1. Cheapest and least maintenance costs. If you don't have many outages--do this, and get a generator (gas, natural gas, propane) for emergency backup
2. Certainly less efficient--makes most sense if no grid available
3. Hybrids are nice, should be more efficient than #2. Basically, a solar powered UPS with Grid Tie inverter.
4. Hombrew:

• a. can be done, switches and/or jack panel. Certainly easy to wire a part of your array (maybe 1/4 or so easily, more of the array requires more work and pre-planning). Issues with code and safety (depending on how you do it).
• b. Also can be done--but at this time, I don't see any good (safe and reliable) way of integrating with a battery charge controller (complex system design issue). Go with #3 if you need a "full" sized GT / Off-Grid system.

-Bill

Bill, can you please explain how the Homebrew option would be done using switches and or jack panel. I want to do something similar and would love to be able to switch a string of my panels from Series to Parallel on the fly with a switch, so I could have lower voltage to charge the battery bank on my golf cart as a emergency backup power solution. I can then put an inverter on the golf cart batteries to power my freezer and fridge ... emergency lighting... renewable emergency energy.

I have a similar Grid-tied system 5.1Kw.

Thanks,
Cabinman

[BB.]

Cabinman,

I will make a suggestion on "how I would do it"... However, there is real danger if you follow my instructions (or not). So, this is just to give you an idea of the basics. The details will be up to you.

As an example... Say you have a two strings of panels. Each panel has a Vmp of 18 volts, there are 10 panels in each string, for a Vmp on your GT inverter of ~360 volts. And each string is 5 amps, for a total of 10 amps to the GT.

Take string "A" and 5 panels in (at the mid point). Break the string and bring two wires down (+ and -), from the "+" of the "left" string, and from the "-" of the "right" string ("left" string, left end is "-" or "ground".

Install three switches (wire switch box near your GT/Solar Charge Controller):

• "I" at the "-" connection to the "A" panel (GT to string)
• "II" at the "+" connection to the "A" Panel Split to DC Solar Charge Controller (5 of 10 panels)
• "III" between the "+" and "-" split terminals.

For normal GT operation... I is closed, II is open, and III is closed. Should be the same as normal GT connections with the current taking a loop down to the switch box and back up... Extra wiring will cost you some power--especially if the runs are long with too small of wire.

Next install:

• switch IV between "-" of A panel to the "-" DC Solar Charge Controller. Connect other end of switch II to "+" of DC Solar Charge Controller.

For normal GT operation, switch "IV" will be open.

So, with switches set as above--power flows through string A as normal.

To switch to DC backup charge controller.

1. Open all switches (I-IV).
2. Then Close Switch IV. (negative connection to solar controller)
3. Then Close Switch III. (positive connection to solar controller)

The above setup will give you 1/4 of your array's rated output (only 5 of 20 panels are connected to your string.

To switch back GT Inverter:

1. Open all switches (I-IV).
2. Close Switch I ("-" to GT inverter)
3. Close Switch II (left and right string back together)

Connections not wired correctly, and/or changing switches in the wrong order will probably ruin one or more pieces of equipment.

Larger arrays will probably have fuses--so you will have to take those into account.

If the above does not make sense, I can try my long rusty drawing skills.

The switches may be real switches, or done with jumpers (and appropriate panel/plugs).

Having emergency solar is a strong draw--but there are serious issues with safety and electrical codes if done wrong (and even if done "right").

Perhaps Solar Guppy can chime in here... He has built exactly this sort of system (using breakers and jumpers?) because he needs to be able to reconfigure his arrays for his various test beds. But--remember--was a very experienced electrical design Solar/RE engineer when he built (and uses) his system.

I know he made a post before with some pictures... But, alas, I did not find it.

Here is one of Solar Guppy's posts describing his "back driving" a True Sine Wave off-grid capable inverter with a GT Inverter (option 4.b from my post (actually works well--but major issues with safety).

-Bill

[Solar Guppy]

It requires that the solar panels for the high voltage sting be home-run'ed every 48V ( Nominal ). So for example if you have 10 24V panels in a string, then you would have 5 sets of wires from the panels to the switch bank. where high voltage would only require a single pair.

The Switchs need to be DPDT , center off. The PV ( plus and minus must both be switched! ) enter on the center commons, the one pole connects to the next and previous switch for series and the other pole connects the + to a breaker and - to a common ground

It works well, but unless you have monster 600V DC rated switchs, care must be taken to only change the configuration when there is no load on the panels, otherwise, they will arc and destory the contacts.

It is very handy, but literally 6X the wiring ( copper isn't cheap ). I only have this setup on two of my 4 arrays for test and design support. To run in an emergency, I'd use the GT inverters to back feed the XW .. much simpler and better efficiency

Thanks guys. Let me think this one through and get back to you ....

Cabinman

Thanks again guys for your responses. I can visualize the jumper / switch solution .. and can see what you mean by safety issues. The XW solution seems good on the surface, but the XW is only rated for 50.4 nominal VDC, which is way too low for even one string of my Sharp 170w panels (about 10 per string). I would be nice to have a controller that was rated for high voltage, low amp, for the design of the off-grid solutions.

I ran this problem by an old electrician I know and he told me that he was not a DC expert, but said I should look at rheostat(s) to see if there might be a fit there. It seems to me that this would be a large business opportunity for the Inverter Folks or some motivated small start-up group. I have blogged a few folks and it seems that there are lots of folks out there looking for backup solutions to their already installed Grid-tied inverters….. want to make a million

Thanks,
Cabinman